Lactic acid is a renewable material obtained by the bacterial fermentation of corn starch or cane sugar, and thus is considered a natural or in otherwords “biomass” material. Lactic acid has two optical isomers: L-lactic acid (also known as (S)-lactic acid) and D-lactic acid (also known as (R)-lactic acid), depicted as follows:

Polyesterification of lactic acid affords polylactic acid polymer.
More typically, lactic acid is typically converted to the cyclic lactide monomer, and the lactide undergoes ring opening polymerization, such as depicted as follows:

The resulting polymer material is typically referred to as polylactide polymer.
The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D and/or meso-lactide to L cyclic lactide monomer used. Likewise, for polymers prepared by direct polyesterification of lactic acid, the degree of crystallinity is largely controlled by the ratio of polymerized units derived from D-lactic acid to polymerized units derived from L-lactic acid.
A nucleating agent can be added to polylactide polymer to accelerate crystallization.